impact of climate change on diffuse pollutant fluxes at the watershed scale

HYDROLOGICAL PROCESSESHydrol. Process. 28, 1962–1972 (2014)Published online 5 March 2013 in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/hyp.9723

Impact of climate change on diffuse pollutant fluxes at thewatershed scale

Mehdi Ahmadi, Rosemary Records and Mazdak Arabi*Department of Civil and Environmental Engineering, Colorado State University, Fort Collins, CO 80523, USA

*Cme805E-m

Co

Abstract:

This study aims to assess watershed-scale impacts of changing climate on sediment, phosphorus, nitrogen and pesticide (atrazine)fluxes over the 21st century at the watershed scale. In particular, changes in dissolved and particulate forms of water qualityconstituents in response to climate change are investigated. The hydrologic model Soil and Water Assessment Tool was calibratedand evaluated in a primarily agricultural watershed in the Midwestern United States to simulate hydrologic and water qualityprocesses on a daily basis over the 2015–2099 time horizon. The model was then driven with 112 distinct statistically downscaledclimate projections representing Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios (IPCC SRES)low, moderate and high greenhouse gas emission scenarios. Projected hydrologic and water quality responses were categorizedaccording to the three IPCC SRES emission scenarios for summarizing and synthesizing results over early-century (2015–2034),mid-century (2045–2064) and late-century (2080–2099) assessment. Results revealed clear warming trends in the study area,whereas small increases in precipitation were predicted. Streamflow, sediment and total nutrient loads did not differ noticeablybetween assessment periods. However, the proportion of dissolved to total nutrients increased significantly from early-century tolate-century periods. With the exception of total atrazine in the mid-century period, predicted pollutant loads for a given assessmentperiod did not differ between emission pathways for a given assessment period. Changes in pollutant fluxes showed pronouncedmonthly variability. The projected increase in readily available forms of nutrients has important implications for the ecologicalhealth of water systems and management of drinking water supplies. Copyright © 2013 John Wiley & Sons, Ltd.

Supporting information may be found in the online version of this article.

KEY WORDS climate change; hydrology; water quality; modelling; nutrients; SWAT

Received 21 August 2012; Accepted 8 January 2013

INTRODUCTION

Non-point source pollution is the primary cause ofimpairment for water bodies in the United States andmany regions around the world. Elevated nutrient,sediment and pesticide loads to waterways may negativelyimpact human health and aquatic ecosystems, increasingcosts of pollutant mitigation andwater treatment (Novotny,2003). Watershed programmes, such as the US Environ-mental Protection Agency Total Maximum Daily Load(TMDL) (NRC, 2001) and the European Water Frame-work Directive (Kaika, 2003), aim to maintain or enhancethe water quality status of water bodies within theirjurisdictions. Achieving the objectives of these pro-grammes necessitates a comprehensive understanding ofpollutant loads into the water body of interest and of thewaste assimilative capacity of the water body. However,the fate and transport of pollutants from diffuse sources andthe ability of water bodies to assimilate pollutants arecontrolled by complex watershed interactions that arelikely to alter substantially with the variability of climaticforcings (Jennings et al., 2009; Novotny, 2003; Zhang,

orrespondence to: Mazdak Arabi, Department of Civil and Environ-ntal Engineering, Colorado State University, Fort Collins, CO23, USA.ail: [email protected]

pyright © 2013 John Wiley & Sons, Ltd.

2009). Characterization of changes in fluxes of water,sediments and chemicals at the watershed scale in responseto the changing climate is essential for the development ofeffective watershed management plans and mitigationstrategies (SWCS, 2003; Tedesco et al., 2005; Garbrechtet al., 2007; Zhang, 2009).Changes in global climate have already been observed

and are well documented (IPCC, 2007). Future climaticchanges are anticipated to alter mean stream discharge aswell as magnitude and intensity of extreme hydrologicevents (Praskievicz and Chang, 2009). Also, evidentshifts in the climatic and hydrologic conditions in awatershed system beget alteration of biogeochemicalprocesses that control the fate and transport of nutrientsand hazardous chemicals (Baron et al., 2009; Solheimet al., 2010). These water quality responses to climatechange are diverse and controlled by multiple factors.Warmer temperatures may intensify loads of somenutrient species to water bodies (e.g. nitrate) whilereducing loads of other constituents (e.g. organic nitrogenand phosphorus) by accelerating decomposition andmineralization and altering timing and magnitude of lowand high flows. Climate-induced changes in frequencyand intensity of precipitation may increase nutrient fluxesvia greater stream discharge and accelerated surfacerunoff and erosion of upland soils or, conversely, mayreduce loads under summer drought conditions andassociated low flows (Solheim et al., 2010).

1963WATERSHED SCALE IMPACTS OF CLIMATE CHANGE ON WATER QUALITY

Hydrologic models coupled with projected futureclimate scenarios are used to investigate hydrologic andwater quality responses to climate change (Praskieviczand Chang, 2009). The majority of modelling studiesassessing these responses to climate change have beenconducted at the level of large river basins or politicalboundaries (states or countries) (Abbaspour et al., 2009;Gassman et al., 2007; Jha, 2004; Krysanova et al., 2005;Marshall and Randhir, 2008; Stone, 2003). Modellinglarge geographical extents necessarily entails configur-ation of larger computational units and assimilation ofdata at coarser spatial resolutions. However, potentialimpacts of the changing climate on water resources mustbe characterized at the watershed scale at which diffusepollution processes function (Chaplot, 2007).Although a large body of hydrologic modelling research

has addressed climate-induced changes in water quantity(Lettenmaier et al., 1999; Wood et al., 2004; Hay et al.,2011), far fewer studies have addressed potentially severeconsequences for water quality (Murdoch et al., 2000;Jeppesen et al., 2009), and an even smaller number ofstudies has characterized potential changes in phosphorus(Baron et al., 2009; Solheim et al., 2010; SWCS, 2003).Also, climate-induced changes in partitioning of diffusepollutants, e.g. relative changes in nitrate versus totalnitrogen (TN) loads or the ratio of dissolved reactivephosphorus to total phosphorus (TP), have received littleattention.Similarly, the response of pesticide fluxes at the

watershed scale to the changing climate has beengenerally overlooked (Bloomfield et al., 2006; Solheimet al., 2010). Impacts of climate change on loads ofhazardous materials such as pesticides to water bodies arecomplex and likely to be highly variable with thecompound and watershed considered. Changes in hydro-climatic controls such as temperature, evapotranspirationand storm frequency may substantially impact pesticideresidence times and delivery ratios, causing climate-induced shifts in pesticide degradation, volatilization andtransport (Bloomfield et al., 2006; Harman et al., 2011).Previous assessments of climate change impacts on

water quality have utilized a relatively narrow range ofpotential future climate scenarios. Climate scenarioconditions are typically incorporated into hydrologicmodels as a combination of changes in temperature andprecipitation and, in some cases, atmospheric concentra-tions of CO2. An emerging body of research utilizesdownscaled climate projections derived from generalcirculation model (GCM) grids. GCMs project a range offuture climates and are typically based on greenhouse gas(GHG) emissions scenarios outlined by the Intergovern-mental Panel on Climate Change Special Report onEmissions Scenarios (IPCC SRES). Most previousresearch utilizing GCMs has selected scenarios from onlya few models (Hanratty and Stefan, 1998; Jha, 2004;Marshall and Randhir, 2008; Abbaspour et al., 2009; Parket al., 2011). However, studies indicate that the selectedclimate scenarios may be the greatest source ofuncertainty in predicting future hydrologic and water

Copyright © 2013 John Wiley & Sons, Ltd.

quality conditions (Gassman et al., 2007; Praskievicz andChang, 2009; Ficklin et al., 2010; Foy, 2010). Use ofemission path ensembles is recommended to removeindividual GCM biases and improve model predictivepower (Hay et al., 2009; Pierce et al., 2009). Ensemblesderived from an extensive set of climatic projectionsprovide critical insight into the range of future hydrologicand water quality responses that may not be adequatelycharacterized when scenarios from only a few GCMs areconsidered.The primary goal of this paper is to assess the potential

watershed-scale impacts of the changing climate onpollutant fluxes over the 21st century, specificallysediment, phosphorus, nitrogen and atrazine. Specificobjectives are (1) to understand climatic conditions undera comprehensive set of 112 climate projections consistentwith IPCC SRES emission pathways and models; (2) tofully enumerate and synthesize hydrologic and waterquality responses to projected climate scenarios and (3) toinvestigate changes in dissolved and particulate waterquality constituents. These objectives are investigated inthe Eagle Creek Watershed (ECW), a primarily agricul-tural watershed in the Midwestern United States. Thepotential impacts are investigated by holding manage-ment and land uses unchanged over the course of the 21stcentury. Stakeholders have identified reductions in nitrateand atrazine loading as critical to water quality improve-ments in the watershed, which is a major source ofdrinking water for the Indianapolis metropolitan area.

METHODS AND MATERIALS

The Soil andWater Assessment Tool (SWAT) was used inan exploratory mode to characterize potential water qualityimpacts of climate change in the ECW, Indiana. TheSWATmodel was calibrated for the period 1995–2000 andevaluated for the period 2001–2004 for daily streamflowand monthly nitrate and atrazine loads. A total of 112statistically downscaled climate projections, representingIPCC SRES emission pathways A2, A1B and B1 and 16GCMs, were used as climatic forcings to the SWATmodelfor 112 distinct simulations over the 2015–2099 period.Hydrologic and water quality responses were categorizedaccording to the three IPCC SRES emission scenarios forsummarizing and synthesizing the results. Temporal trendsin the magnitude of responses were evaluated bycomparing annual time series of responses correspondingto the early-century (2015–2034), mid-century (2045–2064) and late-century (2080–2099) assessment periods.

Study area and data availability

The ECW lies within the Upper White RiverWatershed of central Indiana and drains into Eagle CreekReservoir, which supplies drinking water to the City ofIndianapolis (Figure 1). The 248 km2 basin extends from40�0102400 to 40�0401600 north latitudes and 86�1504300 to86�1604500 west longitudes. Mean annual precipitation forECW is 1052mm and mean monthly temperatures

Hydrol. Process. 28, 1962–1972 (2014)

Figure 1. Location map of the Eagle Creek Watershed and the 2001 land cover map for the 12-digit HUC 051202011102 study subwatershed

1964 M. AHMADI, R. RECORDS AND M. ARABI

(1971–2000) range from �3.3 �C in January to 23.7 �C inJuly (Tedesco et al., 2005). Land use is dominated bycroplands (52%, primarily corn and soybeans) followedby pasture (27%), urban areas (12%) and forest (9%)(Tedesco et al., 2005). Soils are derived from glacialmaterials and generally comprised of thin loess overglacial and alluvial depositions. Much of ECW is poorlydrained, and the majority of the watershed likely containstile-drain systems, which have been shown to be asignificant pathway for pollutant loading to waterways incentral Indiana (Tedesco et al., 2005). Anhydrousammonia and diammonium phosphate are the majorfertilizer types used for corn farming. Phosphorus forsoybean farming is typically provided by application ofdiammonium phosphate. Corn receives 90% of thenitrogen fertilizer and 76% of the phosphorus fertilizerin the ECW (Tedesco et al., 2005). Atrazine is the mostcommonly used pesticide in the region with averageannual application of 1.5 kg/ha. Corn field tillagepractices in the ECW are dominated by conventionaltillage, whereas soybean crop tillage practices aredominated by no-till (Tedesco et al., 2005).Elevated atrazine concentrations have resulted in

atrazine levels in Eagle Creek reservoir in excess of theUS Environmental Protection Agency drinking waterstandard of 3.0 mg/l, and high ECW sediment and nutrientloads have potential to degrade aquatic habitat (Tedescoet al., 2005). A watershed plan has been developed toreduce sediment, nitrate, TN, TP and atrazine loads intothe reservoir (Tedesco et al., 2005). The Eagle Creekreservoir serves as the primary source of drinking waterfor the Indianapolis metropolitan area, and changes inwater quality fluxes in dissolved and particulate forms caninflict substantial cost to water supply utilities.The use of 12-digit hydrologic unit codes (HUCs) is

recommended to encourage participation of stakeholdersin implementing non-point source pollution controlpractices and enable monitoring of water quality changeswithin the subwatershed (Haggard et al., 2010). The 12-digit HUC 051202011102 (41 km2) was selected forinvestigation of effects of climate change in the ECW dueto its contribution to non-point source loads into thereservoir (approximately 23% of nitrate loads and 28% ofatrazine loads). The land use of the subwatershed is 88%


cropland, 6% urban, 4% forest, 1.5% rangeland and asmall fraction of open water (0.5%), according to the2001 USGS National Land Cover Dataset. The locationmap of the ECW and the land use map of the selectedsubwatershed are shown in Figure 1.The meteorological inputs were spatially distributed by

assigning precipitation and temperature values to each ofthe subbasins from the two meteorological stations in andaround the study watershed (Figure 1). Solar radiation,wind speed and relative humidity were estimated usingthe weather generator model (WXGEN) build into theSWAT model (Sharpley and Williams, 1990).

Hydrologic and water quality simulation model

The SWAT (Arnold et al., 1998) was used to simulatehydrologic and water quality processes in the ECW underclimate change scenarios. SWAThas been used successfullyin a large base of international hydrologic and water qualityresearch (Gassman et al., 2007). It is a process-based,distributed-parameter simulation model that operates on adaily or smaller time-step and was developed by the USDepartment of Agriculture to assess impacts of landmanagement in large watersheds over long periods (Arnoldet al., 1998). The model delineates a watershed intosubwatersheds and further subdivides each subwatershedinto hydrologic response units (HRUs), which correspond tounique combinations of land cover, soil types and slopeclasses (Arnold et al., 2011). Major watershed processes ofSWAT include snow accumulation and melt, evapotrans-piration, infiltration and percolation, pond and reservoirstorage, crop irrigation and growth, surface runoff androuting, groundwater flow, and sediment erosion, transportand deposition (Arnold et al., 2011).Soil and Water Assessment Tool has the capacity to

simulate important nutrient and pesticide processes, includ-ing plant use, soil biogeochemical processes and in-streamtransformations. Nitrogen in SWAT is partitioned intomineral (ammonium and nitrate) and organic (active, stableand fresh) fractions. Nitrogen may be removed from anHRU via plant assimilation, leaching, denitrification,volatilization, runoff and soil erosion. Phosphorus in thesoil is partitioned into six distinct pools within mineral andorganic fractions andmay be removed via plant assimilation

Hydrol. Process. 28, 1962–1972 (2014)


or mass flow of water. Pesticides applied to an HRUundergo processes based in part on the GLEAMS model(Leonard et al., 1987). Pesticides may be transported to astream network through surface runoff in either sorbed ordissolved form or leached into the soil profile and aquifer insolution (Arnold et al., 2011). Once in-stream, nutrienttransformations are governed by procedures adopted fromthe QUAL2E water quality model (Brown and Barnwell,1987) while pesticide transformations follow first-orderdecay relationships (Arnold et al., 2011).The SWAT model for the study watershed was set up

using 35 subwatersheds and 446 HRUs using a 30-mresolution digital elevation model from the US GeologicalSurvey National Elevation Dataset (USGS NED, 2010),county-based soil data from the US Soil SurveyGeographic Database (USDA NRCS, 2010), and twogeospatial land cover products: the 1992 and 2001National Land Cover Dataset for non-cropland areas(USGS NLCD, 1992; 2001) and the 1995 and 2000–2004National Agricultural Statistics Service Cropland DataLayers for croplands (USDA NASS, 2003). Land use wasupdated on an annual basis using geospatial land coverproducts and SWAT land use update component.Daily streamflow data at the watershed outlet (US

Geological Survey gauge number 03353200; site 35,Figure 1) and instantaneous nitrate (NO�

3 ) and atrazinesampled on a bi-weekly and monthly basis by the IndianaDepartment of Environmental Management at two sites(sites 20 and 32, Figure 1) were utilized for modelcalibration and evaluation. Monthly loads of nitrate andatrazine were estimated from instantaneous concentrationvalues using the LOADEST model (Runkel et al., 2004).Calibration was performed for the 1995–2000 period,following an initial 2-year warm-up. The model was thenevaluated for daily streamflow and monthly nitrate andatrazine loads for the 2001–2004 period. The parametersof SWAT affecting the streamflow, nitrate and atrazinewere identified after an intensive local and globalsensitivity analysis. Phosphorus and erosion parametervalues were identified through a detailed literature review.Table A1 provides a list of the selected parameters andtheir calibrated values. The simulated and observedstreamflow hydrograph and the atrazine and nitrateloadings were shown in Figure A1.

Climate scenarios

General circulation models are commonly used toderive climate scenarios for hydrologic and water qualityimpacts analysis (Praskievicz and Chang, 2009). GCMclimate predictions typically use IPCC SRES scenariosA2, A1B and B1, corresponding to high, moderate andlow GHG emissions, respectively (IPCC, 2007). BecauseGCMs are typically at too coarse a resolution (spatiallyand temporally) for watershed-scale assessments, they aregenerally bias-corrected and then downscaled prior to useas hydrologic model forcings (Fowler et al., 2007).Climate projections for two meteorological stations

(shown in Figure 1) in the study area were generated


using the following procedure: (1) GCMs and emissionpath scenarios were selected, (2) downscaled climateprojections at a 1/8� spatial grid scale were obtained fromthe World Climate Research Programme’s (WCRP’s)Coupled Model Intercomparison Project phase 3 (CMIP3)multi-model dataset, (3) projected scenarios at 1/8� gridswere further spatially downscaled to remove potentialbias between grid-based projections and meteorologicalstations within the study area and (4) projectedtemperature and precipitation values were temporallydownscaled from monthly to the daily time intervalsrequired by the SWAT model.The CMIP3 dataset (WCRP, 2012) incorporates 25

models and is the product of one of the largest and mostcomprehensive global coupled climate models and multi-model analyses efforts to date. An archive of bias-corrected and spatially downscaled climate projectionsderived from 16 CMIP3 GCMs and served at http://gdo-dcp.ucllnl.org/downscaled_cmip3_projections/, as de-scribed by Maurer et al. (2007) was utilized in this study.Information for each GCM is summarized in Appendix I(Table A2). Data were developed through a collaborativeeffort between the Lawrence Livermore National Labora-tory (LLNL), US Bureau of Reclamation (USBR), SantaClara University and Climate Central and will be referredto as the LLNL/USBR dataset hereafter. The LLNL/USBR dataset contains 112 climate projections andrepresents 16 GCMs developed by 14 modelling groupsfrom 10 countries. For each GCM, there are one or moreruns representing unique initial conditions for IPCCSRES A2, A1B and B1. Each climate projection wasbias-corrected and spatially downscaled using a two-stepprocedure applied to both monthly precipitation andsurface air temperature, and served at a 1/8� grid scale(Maurer et al., 2007).Further spatial and temporal downscaling was required to

implement the climate projections into the ECW SWATmodel. First, using a spatial downscaling technique, whichremoves the bias between LLNL/USBR climate projectionsat 1/8� grid and a meteorological station location within thegrid, it was ensured that projections of precipitation andtemperature were consistent with observations over the1950–2010 period, when data were available. Observationsof precipitation and temperature from meteorologicalstations in and around the study watersheds were comparedwith the LLNL/USBR data from the corresponding grid cellencompassing the station, and the bias between the two wasremoved through scaling (precipitation) and incrementing(temperature) on a monthly basis. The scaling factor (ci, j)for removing precipitation bias at climate station j wascomputed separately for the ith month of the year (i=1,2, . . ., 12) as follows: (1) compute observed monthlyprecipitation time-series values from historical dailyrecords, (2) select months corresponding to the ith month(e.g. there are 61 January months in the observed datasetover the 1950–2010 period) and (3) compute the scalingfactor as the ratio of the selected observed average monthlyprecipitation (P) over the corresponding projected values(P̂ ) for the LLNL/USBR grid:

Hydrol. Process. 28, 1962–1972 (2014)

http://gdo-dcp.ucllnl.org/downscaled_cmip3_projections/

http://gdo-dcp.ucllnl.org/downscaled_cmip3_projections/


ci; j ¼XN

m¼1Pmi; j=

XN

m¼1P̂i; j

m (1)

where N is the number of observation years (here, N=61).This scaling factor was then applied to all future

months (k = 1, 2, . . ., 85) corresponding to the ith monthof the year over the 2015–2099 period from LLNL/USBRgrid:

P~k

i; j ¼ ci; j � P̂i; jk

(2)

where P~ represents the bias-corrected projected precipi-tation value for future month k.Similarly, a single increment (di, j) for temperature

corresponding to the ith month of the year at station j wasapplied to the projected future temperature values asfollows:

T~ki;j ¼ T^ i; j

k þ di; j;where di; j ¼XN

m¼1Tmi; j �

XN

m¼1T^ i; j

m(3)

where T~ denotes the bias-corrected future monthlytemperature for future month k (�C), T is the projectedvalues from LLNL/USBR dataset (�C) and T representsthe observed monthly temperature values over the Nobservation years.Following the bias-correction method, a dataset con-

sisting of average monthly values of precipitation (mm/day) and temperature (�C) at each meteorological stationover the 2015–2099 period was available. An existing re-sampling technique (Bureau of Reclamation, 2009;Maurer et al., 2007; Wood et al., 2002) was modifiedto temporally downscale bias-corrected monthly values ofprecipitation and temperature into daily values ofprecipitation, maximum temperature and minimumtemperature. This method creates daily values of climaticvariables that both preserve observed spatial and temporalcorrelations and aggregate to future monthly projections.The basic premise of this technique involves movingthrough a number of meteorological stations’ simulatedprojections (2015–2099) and associating a randomlyselected corresponding historical month with each monthsimulated in the future. The historical month’s daily seriesare then scaled (precipitation) and incremented(temperature) to create a future month’s daily series thatsatisfy the future monthly projection (Foy, 2010):

P̂j;k ¼P~kPl

� Pj;l (4)

where P^ j;k is the estimated future precipitation on day j offuture month k (mm), P~k is the bias-corrected averagedaily precipitation in future month k (mm/day) fromEquation (2), Pl is the observed average daily precipita-tion in resampled historical month l (mm/day) and Pj,l isthe observed precipitation on day j of resampled historicalmonth l (mm).A similar approach was taken for computing daily

temperature values from projected monthly temperaturevalues:


T̂ j;k ¼ T~k �TlÞ þ Tj;l�

(5)

where T̂ j;k is the estimated future temperature on day j offuture month k (�C), T~k is the bias-corrected projectedaverage daily temperature in future month k (�C) fromEquation (3), Tl is the observed average daily temperaturein resampled historical month l (�C) and Tj,l is theobserved temperature on day j of resampled historicalmonth l (�C). Further information on the downscalingprocedure is detailed in Foy (2010).

Watershed modelling scenarios

The resulting bias-corrected, spatially and temporallydownscaled daily temperature and precipitation values foreach of the 112 climate projections were then used asforcings in the calibrated and tested ECW SWATmodel toassess impacts of climate change on annual climate,streamflow and water quality fluxes at the outlet of theHUC-12 051202011102 (outlet 20 in Figure 1). Changes inCO2 levels were not included in the modelling scenarios,mainly because SWAT uses a constant CO2 level for eachsubbasin for the entire simulation period. Each of the 112distinct SWAT simulations was run on a daily time-step toproduce average annual outputs for 85 years of simulation(2015–2099). For each of the 112 simulations, averageannual outputs were calculated for early-century, mid-century and late-century assessment periods (2015–2034,2045–2064 and 2080–2099). These 20-year averageannual values were then grouped by the three IPCC SRESGHG emission pathway scenarios (A2, A1B and B1), andemission pathway ensemble means, medians and quartilesof 20-year means were computed for the assessmentperiods. One-way analysis of variance (ANOVA) was usedto statistically test the significance of changes in hydrologicand diffuse pollutants fluxes amongst the assessmentperiods. Analysis of covariance was used to test thesignificance of differences between emission pathwayensembles in soluble nutrient responses to temperature.

RESULTS AND DISCUSSION

Results indicate clear warming trends over the course ofthe 21st century with less substantial increases inprecipitation for this Midwestern US study watershed.Streamflow, sediment and total nutrient loads did notdiffer between assessment periods. However, the propor-tion of dissolved to total nutrients increased, with nitrateand soluble phosphorus yields increasing significantlybetween early-century and late-century. With the excep-tion of total atrazine in the mid-century, yield of waterquality constituents was not significantly differentbetween emission pathway ensembles for a givenassessment period. Changes in diffuse pollutant fluxesshowed pronounced monthly variability, although differ-ent trends were evident for nutrients and pesticides.Increases in future loads of nitrogen and phosphorusoccurred primarily during winter and the growing season,whereas atrazine loads were predicted to increase insummer and fall but decrease in winter and spring.

Hydrol. Process. 28, 1962–1972 (2014)

Figure 2. Predicted precipitation and temperature in Eagle CreekWatershed for 112 distinct climate projections for B1, A1B and A2representing low, intermediate and high Intergovernmental Panel onClimate Change Special Report on Emissions Scenarios (IPCC SRES)greenhouse gas emission scenarios, respectively. The box and whiskerplots represent the minimum, maximum, upper and lower quartile, andmedian of period averaged annual values within each IPCC SRESemissions scenario. The dashed line represents the average annual

(baseline) value over the period 1986–2005


Model calibration and evaluation

Calibration of the SWAT model was performed fordaily streamflow at the ECW outlet (site 35, Figure 1) andmonthly nitrate and atrazine observations at two sites(sites 20 and 32, Figure 1) for the period 1995–2000,following an initial 2-year warm-up. The model was thenevaluated for daily streamflow and monthly nitrate andatrazine loads for the period 2001–2004. Error statisticsused for evaluating the performance validity of the modelincluded percent bias, coefficient of determination andNash–Sutcliffe efficiency coefficient. The Nash–Sutcliffeefficiency statistic is a normalized statistic that charac-terizes the fit of observed versus simulated data on a 1:1line. It ranges from �1 to 1.0, with 1.0 indicating aperfect fit. Table I summarizes the performance criteriafor streamflow, nitrate and atrazine loads over thecalibration and evaluation periods. In general, the modelyielded very good to satisfactory statistics for streamflowand nitrate in the calibration and evaluation periods, onthe basis of performance criteria outlined by Moriasi et al.(2007). It was also evident that the SWAT modeladequately represented atrazine fluxes in the watershed.The LOADEST regression model was used to estimatemonthly loads of nitrate and atrazine from instantaneousconcentration values. Although LOADEST was deemedsatisfactory in many studies, a few studies raised concernsabout the adequacy of model when residuals of regressionequations are heteroscedastic (Stenback et al., 2011).

Precipitation and temperature trends

Box plots in Figure 2 present the variability in projectedaverage annual precipitation and temperature values overthe three assessment periods summarized according to thecorresponding IPCC SRES ensemble. Substantial increasesin both minimum and maximum average annual tempera-tures under all emission pathway scenarios were evident.One-way ANOVA showed that mid-century and late-century temperature projections are significantly higherthan early-century projections (p< 0.001). Changes inprecipitation were generally less pronounced than thosefor temperature but indicated an overall modest increase inannual precipitation, consistent with predictions for otherwatersheds in the American Midwest (Jha, 2004; Van Liew

Table I. Calibration and evaluation performance criteria formonthly nitrate, monthly atrazine and daily streamflow

Gaugingstation Variable

Calibration period(1995–2000)

Evaluation period(2001–2004)

PBIAS(%) R2 NSE

PBIAS(%) R2 NSE

20 Nitrate 7.9 0.94 0.83 16.9 0.85 0.67Atrazine �6 0.81 0.34 �14 0.7 0.41

32 Nitrate �7.9 0.92 0.84 8.4 0.76 0.55Atrazine 42.3 0.75 0.52 33.1 0.51 0.14

35 Streamflow �12.2 0.78 0.61 4.3 0.78 0.56

PBIAS and NSE are percent bias between simulated and observed valuesand Nash–Sutcliffe Efficiency, respectively.


et al., 2012; Woznicki and Nejadhashemi, 2012). Mediansof 20-year average precipitation for late-century emissionpathway ensembles increased above early-century projec-tions by 4%, 7% and 10% for B1, A1B and A2 scenarios,respectively. Larger ranges between lower and upperwhiskers in the late-century suggest the higher uncertaintyin prediction of climate conditions at the end of the century.One-way ANOVA of projected precipitation showed thatthe late-century predictions of precipitation are significantlyhigher than early-century predictions (p< 0.01). However,mid-century predictions did not show significant differencesfrom early-century and late-century predictions.

Stream discharge and sediment loads

Although median annual streamflow and sedimentyields appeared to show small upward trends over time,changes over the next nine decades under any givenemission scenario were neither significant nor did theyvary significantly between emission pathway ensemblesfor a given assessment period (ANOVA, p> 0.05).Figure 3 shows predictions of stream discharge and

Figure 3. Predicted streamflow and sediment yield at hydrologic unit code051202011102 outlet in Eagle Creek Watershed for 112 distinct climateprojections for B1, A1B and A2 representing low, intermediate and highIntergovernmental Panel on Climate Change Special Report on EmissionsScenarios (IPCC SRES) greenhouse gas emission scenarios, respectively.The box and whisker plots represent the minimum, maximum, upper andlower quartile and median of period averaged annual values within eachIPCC SRES emissions scenario. The dashed line represents the average

annual (baseline) value over the period 1986–2005

Hydrol. Process. 28, 1962–1972 (2014)


sediment yield at the HUC 051202011102 watershedoutlet. Increases in streamflow and/or sediment yieldsunder future climate have been predicted in severalMidwestern watersheds (Jha, 2004; Van Liew et al.,2012; Stone, 2003; Woznicki and Nejadhashemi, 2012),with the exception of a fewMidwestern basins where meanannual decreases in discharge are forecasted (Hay et al.,2011). However, significance of such changes hasgenerally not been evaluated, with some exceptions (Hayet al., 2011), limiting the applicability of comparison toresults of other regional assessments.

Nitrogen and phosphorus loads

Significant changes over the analysis periods for all waterquality constituents were evident with the exception of TNand TP yields (Figure 4). Nutrient yields (TN, TP, nitrateand soluble phosphorus) showed no significant differencebetween emission pathway ensembles for a given assess-ment period (p> 0.05). Although TN and TP yields did notchange significantly over the three assessment periods for agiven emission pathway, dissolved nutrient loads showed amarked increase between early-century and late-centuryperiods, resulting in modest percent increases in average

Figure 4. Predicted pesticide and nutrient yields at hydrologic unit code 0projections for B1, A1B and A2 representing low, intermediate and high IScenarios (IPCC SRES) greenhouse gas emission scenarios, respectively. Thequartile, and median of period averaged annual values within each IPCC S

(baseline) value over th


annual nitrate loads (a maximum of 10% increase in themedian of assessment period 20-year average loads underensemble A2, p< 0.001) and large increases in averageannual soluble phosphorus loads (97–129%, correspondingto low to high GHG emission pathway ensembles,p< 0.001). ECW fertilizer application rates were un-changed between the baseline period and climate scenarios,and therefore, the nitrogen and phosphorus available forloading to streams would be expected to be similar in futurescenarios. Previous studies of experimental watersheds inthe United States suggest that nitrogen and phosphorusexports from agricultural basins may exhibit relativelyunchanging concentration despite large streamflow vari-ability (chemostatic responses), particularly in basins with alegacy of agricultural fertilizers such as ECW (Thompsonet al., 2011). This suggests that the similar total nutrientloading predicted between assessment periods is a reflectionof actual ECW conditions rather than an artefact ofmodelling.Changes in the dissolved nutrient fractions are of great

interest. Such changes may represent relatively smallabsolute increases in annual average yields but indicate aconsiderable climate-induced change of nutrient partitioningin the watershed. The warmer, wetter conditions predicted

51202011102 outlet in Eagle Creek Watershed for 112 distinct climatentergovernmental Panel on Climate Change Special Report on Emissionsbox and whisker plots represent the minimum, maximum, upper and lowerRES emissions scenario. The dashed line represents the average annuale period 1986–2005

Hydrol. Process. 28, 1962–1972 (2014)

Figure 5. Correlation between average annual temperature and soluble phosphorus at hydrologic unit code 051202011102 outlet in 112 distinct climateprojections for B1, A1B and A2 representing low, intermediate and high Intergovernmental Panel on Climate Change Special Report on EmissionsScenarios greenhouse gas emission scenarios, respectively. All linear regression models were statistically significant with coefficient of determination

greater than 0.9 and p-values less than 0.001

Figure 6. Modelled monthly changes in diffuse pollutant changes in theEagle Creek Watershed from control period (1986–2005) to mid-centuryperiod (2045–2064). The box and whisker plots represent the minimum,maximum, upper and lower quartile, and median of period averagedannual values within each Intergovernmental Panel on Climate Change

Special Report on Emissions Scenarios emissions scenario


under future climate for the ECW will tend to increasedecomposition of organic matter and mineralization ofnitrogen and phosphorus (Murdoch et al., 2000), processeswhich are strongly dependent on water availability andtemperature in SWAT (Arnold et al., 2011). Such trends havebeen noted in several previous hydrologic modelling studies.For example, Bouraoui et al. (2002) and Boorman (2003)attributed predicted increases in total nitrate and solublephosphorus losses in the Yorkshire Ouse basin of northernEngland in part to accelerated mineralization of biomass.Similarly, Shrestha et al. [2012] attributed increases in TNand phosphorus loads to enhanced decomposition andmineralization of organic matter in the Upper Assiniboinecatchment ofManitoba, Canada. The strong relation betweenaverage annual temperature and dissolved nutrient fractions isto be expected. However, the sensitivity of solublephosphorus to temperature in early spring and growingseason appears to vary with the emission pathway ensemble,with greater slopes corresponding to lower GHG emissionpathway scenarios (Figure 5a). This may be due to greaterrainfall and moisture availability at lower GHG emissionlevels. Nevertheless¸ sensitivity of soluble phosphorus totemperature did not vary significantly with the emissionpathway ensemble inwinter (p> 0.01), as shown inFigure 5b.The changes in total diffuse pollutant loads showed

pronounced monthly variability. Box plots in Figure 6show the projected monthly changes in TN and TP at thewatershed outlet from control period (1986–2005) to mid-century period (2045–2064). The increases in TN and TPloads were higher in late winter, early spring (March toApril) and growing season (June to August). Theincreased winter loads are likely due to greater runofffrom upland soils. However, increased loads during thegrowing season can be related to increased decompositionof total organic matters due to elevated temperature(shown in Figure 5). Relatively constant TP loads asopposed to the increased TN and atrazine loads in July arelikely due to the lower erosion and runoff that are themain factors in transport of phosphorus from uplands andstream banks to the surface waters. The projected TP andTN loads decreased relative to the control period in latesummer and early fall (September to November), whenrunoff generally decreases, whereas they showed an


increasing trend in late fall and early winter (November toDecember), with higher increases in TN loads. This canbe explained by tillage operations practiced in Novemberthat subsequently increase the nitrogen and phosphorusleaching into tile drains. Changing the fertilizer applica-tion timing and replacing chemical fertilizer with manureor slurry can potentially abate the risk of increasednutrients loads (Solheim et al., 2010).

Hydrol. Process. 28, 1962–1972 (2014)


Atrazine loads

Total atrazine loads were predicted to increase betweenearly-century and late-century and between mid-centuryand late-century assessment periods under the high GHGemission ensemble (A2) (p< 0.05). Other GHG emissionensembles showed no significant difference betweenassessment periods. With the exception of total atrazinein the mid-century, yield of water quality constituents wasnot significantly different between emission pathwayensembles for a given assessment period. Increase inyields of pesticides with greater future precipitation isintuitive and has been predicted in a previous study(Ficklin et al., 2010), although for different compoundswith distinct properties. Variability and thus uncertaintyin total atrazine predictions increased with time (as shownby the wider range between upper and lower whiskers,Figure 4), particularly for the A1B scenario in the late-century period.Partitioning of atrazine in SWAT is a function of soil

adsorption coefficient (Koc) and soil organic carbon contentand is not directly dependent on temperature or precipitation;the proportion of soluble atrazine yield did not altersubstantially under future climate predictions (Figure 4).Atrazine is moderately water soluble. Because it does nottypically adsorb strongly to sediments and has a relativelylong half-life, it is mobile in soils (Cornell UniversityExtension, 1993). Degradation and volatilization of atrazinein SWAT in HRUs and in-stream is modelled withexponential decay functions controlled largely by com-pound-specific half-life values (Arnold et al., 2011). Increasesin predicted total atrazine under the A2 emission pathwayensemble are therefore likely not a product of climate-inducedincreases in microbial activity. If runoff events are morefrequent in the future, it is possible that atrazine may undergoless degradation prior to in-stream transport, resulting ingreater total atrazine yields (Solheim et al., 2010).Changes in atrazine loads showed somewhat different

monthly variability from changes in nitrogen and phos-phorus fluxes (Figure 6). Box plots in Figure 6 show theprojected monthly changes in total atrazine at thewatershed outlet from control period (1986–2005) tomid-century period (2045–2064). The total atrazine loadswere predicted to increase in summer and fall but wereprojected to decrease in winter and spring. Increased loadsof atrazine in summer can be related to the increases inupland runoff during the growing season when pesticidesare applied. Pesticide application usually starts at plantingdate, which is April or May in the ECW. Following thisincreased loss of atrazine, less atrazine would be availableto enter the stream network during the winter.

SUMMARY AND CONCLUSION

The need to incorporate climate change impact assess-ments into regulatory programmes such as the TMDL andWater Framework Directive has been recognized in anumber of studies and watershed management plans(Tedesco et al., 2005; Zhang, 2009). However, the smallnumber of climate scenarios typically used may result in a


narrow range of projections for future hydrologic andwater quality fluxes that are strongly influenced bypredictive errors from the selected GCMs (Pierce et al.,2009). In this paper, changes in hydrologic and pollutantfluxes driven by an extensive ensemble of climateprojections were analysed at the watershed scale overthe 21st century. The SWAT was used to characterizepotential water quality impacts. The direction andmagnitude of changes were analysed by combining 112distinct climatic projections and computing IPCC SRESemission scenario ensemble median and quartiles forstreamflow, sediment, nitrogen, phosphorus and atrazinefor early-century, mid-century and late-century assess-ment periods (2015–2034, 2045–2064 and 2080–2099).Annual warming trends were apparent for temperature

in the study watershed, with significant but smallincreases in precipitation. Streamflow and sediment yieldsdid not differ between assessment periods. All predictionsgenerally became more variable and thus more uncertainover time. Although the wide range of predicted waterquality responses in this study demonstrates the largeuncertainty associated with climate projections, the use of112 distinct projections provided critical insight intopotential future median and extreme water resourcesconditions. Analysis of climate projections grouped byemission pathway ensembles also revealed that waterquality predictions generally did not vary between GHGemission levels for a given assessment period.The average annual yields of TN and TP tended to remain

relatively unchanged over the three assessment periods forany given emission category, whereas total atrazineincreased only between certain assessment periods for thehighest GHG emission pathway ensemble. However, theproportion of dissolved (nitrate and soluble phosphorus) tototal nutrients generally increased from the early-century tothe late-century periods. This increase in readily availablenutrients has important implications for long-term manage-ment of reservoirs and drinking water supplies, as well asecosystem processes within the watershed.In many agricultural watersheds such as the ECW,

increases in dissolved, readily available forms of nutrientsmay induce or exacerbate eutrophication in receiving waterbodies, particularly under future warmer air temperaturesand associated lower dissolved oxygen levels (Murdochet al., 2000; Zhang, 2009). Eutrophic reservoirs such as thatin the ECW commonly contain algal blooms and associatedtoxins, posing a challenge for protection of drinking waterresources even under present climatic conditions (Tedescoet al., 2005). Additionally, elevated nitrate levels in drinkingwater are associated with methemoglobinemia, or ‘bluebaby syndrome’, and may pose a direct threat to drinkingwater quality (Carpenter et al., 1998). Results of this studysuggest that watershed managers may need to account forpotential climate-induced changes not only in dissolvednutrient fractions but also in pesticide yields to allocate loadseffectively for the long term.Findings presented here yield important insights into

potential future hydrologic analysis and water quality fluxesat the watershed scale. However, several aspects merit

Hydrol. Process. 28, 1962–1972 (2014)


further investigation. By holding management and land useconstant throughout climate scenario assessment, we haveinvestigated the potential mean annual hydroclimatic anddiffuse pollutant impacts andmonthly nutrient and pesticideeffects of climatic change under baseline conditions.However, earlier snowmelt, warmer air temperatures and alonger growing season predicted formuch of NorthAmericaand urbanization have the potential to substantially alter landuse and management from historical patterns. Such changesmay affect future diffuse pollutant fluxes, mitigating orexacerbating impacts of climate change on water quality(Praskievicz and Chang, 2009). Investigation of theinteractions of climate, land use and management may alsoyield insight into how effectiveness of soil and waterconservation practices targeted at water quality improve-ments may change or be optimized under future climate(Nicklow et al., 2010; Woznicki and Nejadhashemi, 2012).Additionally, GCM-based projections and hydrologicmodels utilizing GCM inputs represent a range ofuncertainty in future hydroclimatic conditions, but changesbeyond this range are possible (Brown and Wilby, 2012).Model performance criteria indicate that the hydrologicmodel adequately represents hydrologic and diffuse pollu-tant dynamics within the watershed (Table I). However, useof the SWAT model to climate impact assessments in thefuture may be strengthened by improvements to land unitconnectivity, stream channel erosion and sediment depos-ition processes, event-based flood and sediment routing, andsimulating deep aquifer contribution on baseflow. Finally,assessment of potential future hydroclimatic changes on amonthly basis would enhance understanding of seasonalchanges in snowfall, peak and low flows. Such assessmentsmay provide valuable insight into potential changes toseasonal water availability and alterations in low flow andcritical conditions used in programmes such as TMDL.

ACKNOWLEDGEMENTS

This studywas funded by theUSDepartment of Agriculture–National Institute of Food and Agriculture grants 2007-51130-03876 and 2009-51130-06038. We acknowledge themodelling groups, the Program for Climate Model Diagnosisand Intercomparison (PCMDI) and the WCRP’s WorkingGroup on Coupled Modelling (WGCM) for their roles inmaking available the WCRP CMIP3 multi-model dataset.Support of this dataset is provided by the Office of Science,US Department of Energy.

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